Optical fiber communications have been a commercial reality now for over twenty years. Research and development efforts still continue to explore new laser sources, new forms of optical amplifiers, novel materials for low-loss optical fibers, and the like. At present, long-haul fiber optic communication systems employ small core (on the order of a few microns) single-mode fibers used in conjunction with laser diodes also operating in their single-mode regime, typically the lowest order transverse mode. This configuration yields low intermodal dispersion and allows for what is commonly called dense wavelength multiplexing (DWDM).
Today, low-loss single mode fibers and laser diodes that operate near optimum wavelength for low loss propagation are readily available. Even so, in-line optical amplifiers (all-optical repeaters) are necessary approximately every 400 kilometers (≅250 miles) or so for long haul applications. One such in-line optical booster is the erbium-doped fiber amplifier. This device is capable of simultaneously amplifying multiple wavelengths (multiple optical channels) by optically pumping from a secondary pump laser operating at a shorter wavelength. Notwithstanding these advances, research is still underway to increase the distance between optical repeaters. One approach is to increase the optical energy from the pump laser coupled into the fiber. Practical challenges arise from this approach. First, laser diodes driven at high input currents may transition from single-mode to multimode operation, indicative of the so-called “kink effect” in the light output versus injection current characteristic. Although the laser may have more total output optical energy in the multi-mode state, there are at least two negative drawbacks to this straightforward approach. First, when lasing in the multi-mode regime, the optical energy is less confined near the optical axis of the laser and more divergent as the beam exits the laser end-face. Given this, the coupling efficiency for focusing the laser output into a fiber is reduced. Secondly, the multi-mode operation of the laser may increase the intermodal dispersion in the fiber so as to decrease the available bandwidth for multichannel operation.
Given the above, there is a need for a technique to operate laser diodes at increased optical output levels while constraining the laser to continue to operate in the single-mode regime.